With the wide field of the Mosaic imager, one should be aware of the special conditions imposed by differential atmospheric refraction during exposures. [This section was motivated by Gary Bernstein.]
Figure B.1: This plot shows the differential prismatic distortion due to atmospheric refraction over the extent of the Mosaic field. The difference in angular displacement of a star between the center and corner of the Mosaic array is shown as a function of airmass for the 4m and 0.9m cameras Note that this effect does not vanish at the zenith, as even the angular extent of the Mosaic field at the zenith is large enough for differential prismatic distortion to be present. The particular curves shown are for an STP atmosphere. The actual displacement will vary with temperature, and in general will be slightly lower, given the altitude of KPNO or CTIO. The prismatic distortion will also occur only parallel to a vector pointing to the horizon; the "center-to-corner'' displacement is thus meant only to be representative of typical angular distances within the Mosaic field. Note that as a Mosaic field transits a range of airmasses, as might occur in a dither sequence, the differential prismatic distortion will vary over the sequence.